The present invention relates to laser diode drivers and modulator drivers for fiber-optic communication applications and more particularly to laser diode drivers with output matching networks to improve power efficiency.
U.S. Pat. No. 5,721,456 to N. Kebukawa, assigned to Mitsubishi Denki Kabushiki Kaisha, discloses an optical transmitter in which a light-emitting device is driven by a differential circuit. FIG. 1 shows an optical transmitter with a reflection absorbing circuit 107 coupling between a common-emitter differential amplifier and a laser diode 105. The impedance of the differential amplifier is approximately that of an open circuit while the impedance at the laser diode end is that of a very low resistor, when the laser diode 107 is activated by the biasing circuit 106. There exists a significant mismatch at the interface resulting in considerable reflection. The reflection absorbing circuit 107 can absorb the reflection due to the mismatch between the high impendence differential amplifier and the low impedance laser diode 105. This approach provides a high efficiency means of delivering the modulation current to the laser diode 105.
However, in most modern laser diode modules, there is a matched termination as well as a RF choke for external biasing. Moreover, the integration of an optical transmitter is normally comprised of a laser diode module, a laser diode driver, which is in die format or in package format, and a matched transmission line at the interface. Therefore, with the typical configurations, there is no mismatch at the laser module or at the transmission line. Conventional laser diode drivers are designed with their output impedance matched to the transmission line and the laser diode module. Nevertheless, there is considerable power consumption associated with the matched load inside the laser diode driver. A laser diode driver with a high output impedance can increase the efficiency of delivering modulation current to the matched laser diode module at the expense of mismatch interference between the laser diode driver and the transmission line.
In the article xe2x80x9cA 10-Gb/s Laser Diode/Modulator Driver IC With a Dual-Mode Actively Matched Output Bufferxe2x80x9d, IEEE Journal of Solid-State Circuits, vol. 36, No. 9, Sept. 2001, pp. 1314-1320, H. Ransijn, et al., disclose an active circuit that can be employed to reduce mismatching at the interface as demonstrated illustrated in FIG. 2. A unity-gain matching amplifier 202, MA, with an output impedance 204, Rm, is coupled between the output 206, Vo, with a modulation current 208, Io, driving the load 210, Zo, and a dependent source 212, Voxe2x80x2, which is a combination of a fractional current source 214, Io/k, and a multiple impedance 216, kxZo. Also shown in FIG. 2 is an offset 218, VD, and a terminating impedance 220, RL. Depending on the applications, the driver with this actively matched buffer can provide a DC-coupled back termination to optimize the matching condition at the interface between the output of the laser diode driver and laser diode module. Due to the matching amplifier 202, MA, the mismatch at the interface can be minimized without sacrificing the transmitting power. Therefore, the power efficiency of the output buffer is virtually twice that a conventional driver. However, because of the unknown external offset voltage of the laser diode, this approach requires a DC control loop to be robust to the variation in laser turn-on voltage. This DC control loop results in extra components and less output swing.
To achieve high-speed application, a differential amplifier with cascode configuration is utilized as an output stage of the laser diode driver. In order to reduce the power consumption, a high-impedance resistor is employed as a load for output transistors. The value of the load resistor is higher than the matched resistance of the laser diode module. The high-impedance transistor load ensures most of the modulated current from the transistors flows through the laser diode rather than through the load itself. Nevertheless, without additional circuitry, the mismatch of impedance can induce significant interference at the interface. Instead of a direct connection between the laser diode module and the transistor output, a matching network is employed. The matching network can be a combination of lumped elements, e.g., resistors and capacitors, as well as distributed elements, e.g., transmission lines. Because the mismatch interference is significant at high frequencies, the matching condition between the laser diode and the transistor output can be optimized to reduce high-frequency mismatch by introducing a capacitor in series with a resistor. The in-series resistor is selected to be close to matched resistance of the laser diode module. The matching network with a high DC load, a matched RF load in series with a capacitor, and distributed elements, can significantly reduce the mismatch interference between the high-impedance at the output of a cascode stage and the matched impedance at the laser diode module.
Furthermore, as opposed to Ransijn, et al., an AC-coupled active load is utilized to replace the RF matched resistor and thereby further increasing the power efficiency as compared to a passive matching network. The AC-coupled active matching network obviates the need for a DC control loop and can potentially increase the output swing experienced with the teachings of Ransijn, et al.